This invention relates to a system for control of the air/fuel ratio of an air/fuel mixture being supplied to an internal combustion engine, and more particularly to a device provided in such system for determining the timing of initiation of the air/fuel ratio control operation of the system.
A system is already known which is arranged to effect feedback control of the air/fuel ratio of a mixture being supplied to an internal combustion engine which has a three-way catalyst provided in its exhaust system, in response to an output signal of an exhaust gas ingredient concentration sensor provided in the same exhaust system. As the above sensor, an O.sub.2 sensor is used in general which comprises a sensor element made of zirconium oxide and is adapted to detect the concentration of oxygen present in the engine exhaust gases.
The principle of the O.sub.2 sensor is utilization of a change in the conduction rate of oxygen ions through the interior of the zirconium oxide, which corresponds to a change in the difference between the oxygen partial pressure of the air and the equilibrium partial pressure of the oxygen in the exhaust gases. The internal resistance of the O.sub.2 sensor is determined by the oxygen conduction rate through the zirconium oxide, so that the output voltage produced by the sensor varies as a function of the oxygen concentration in the exhaust gases. Further, the internal resistance of the O.sub.2 sensor also varies with a change in the degree of activation of the sensor. Thus, for accurate air/fuel ratio feedback control, the feedback control operation has to be initiated only after the O.sub.2 sensor has become fully activated.
On the other hand, in an internal combustion engine in general, a choke valve is usually provided in an air intake of the carburetor for supplying the engine with a rich mixture at the cold start of the engine or on like occasions. If the choke valve is an automatic choke which is automatically opened or closed, depending upon a change in the engine temperature, the choke valve is closed at the start of the engine when the engine temperature is low, to supply the engine with a rich mixture. However, if air/fuel ratio control operation is carried out at the start of the engine when the choke valve is still closed, the choke valve cannot exhibit its proper function, since the air/fuel ratio of the mixture being supplied to the engine is then controlled to a value nearly equal to the theoretical air/fuel ratio. Therefore, it is desirable to initiate the air/fuel ratio control operation after the engine temperature, which forms the factor for actuation of the choke valve, has risen to a predetermined value.